Dielectric cavity stripline coupler

ABSTRACT

A three section stripline coupler is disclosed wherein the conducting strips of the center section overlie each other and those of the end sections are displaced sideways relative to each other. Cavities are formed in the ground planes of the stripline over the center section and the cavities are filled with dielectric material to increase the coupling between the conducting strips at the center section.

United States Patent 1191 Friend et al.

[ 1 Oct. 23, 1973 Uberbacher et al 333/10 DIELECTRIC CAVITY STRIPLINE 3,460,069 8/ COUPLER 3,478,281 11/1969 Jeong 333 10 3,113,277 12/1963 Casper et al 333/10 1 Inventors! Lawrence Friend, Scottsdale, 3,063,026 11 /1962 McFarland 333 10 Ariz.; Thad Stevens, Richardson, 3,363,201 1/1968 lsaacson 333/10 Tex, 3,617,952 11/1971 Beech 333/10 [73] Assignee: Motorola, Inc., Franklin Park, 111. Primary Examiner kudolph V Rolinec [22] Filed: June 7, 1072 Assistant ExaminerMarvin Nussbaum [2H AppL No: 260,501 Attorney-Foorman L. Mueller et al.

1 [57] ABSTRACT [52] U.S. Cl..-. 333/10, 333/83 R, 333/84 M I 51 Int. Cl HOlp 5/14, HOlp 3/08 A three "P dlscmsFd [58] Field of Search 333/10, 11,83 R, the the center Ver|1e 333/84, 84 M each other and those of the end sections are displaced sideways relative to each other. Cavities are formed in [56] References Cited the ground planes of the stripline over the center sec- 7 UNI-TED STATES-PATENTS t1011 and the cav1t1es are filled with dlelectnomaterlal v to mcrease the couplmg between the conductlng stnps 3,121,848 2/1964 Kruse, Jr. et a1. 333110 at the center section p 1 5 3,221,275 11/1965 Wambsganss 333/10 I 3,626,332 12/1971 Barbatoe 333/10 5 Claims, 5 Drawing Figures 0 (:5 c D c o 1 DIELECTRIC CAVITY STRIPLINE COUPLER BACKGROUND OF THE INVENTION This invention relates to microwave stripline couplers, more particularly to directional dielectric cavity stripline couplers wherein a substantially flat response is achieved over a greater bandwidth than heretofore, and it is an object of the invention to provide an improved stripline coupler of this nature. The invention further relates to a method or procedure for adjusting the coupling coefficient between the two transmission lines of a microwave stripline coupler.

Directional couplers of the nature here involved for the microwave region and for stripline constructions are known and are commonly referred to 3dB directional couplers. The latter has reference to the fact that one-half of the power in the transmitting circuit is transferred to the receiving circuit and flows in only one direction therein.

Stripline couplers involve two strip conductors separated by a thin dielectric layer and disposed between two ground planes, a dielectric member also being disposed between-each of the strip conductors and the adjacent ground plane. In such constructions the thickness of the dielectric layer and the spacing of the ground planes from the strip conductors (thicknessof the dielectric members) are factors in the coupling coefficient. Couplers include more than one coupling section, typically an odd number of three or more. Also in such couplers, other microwave circuitry may be associated with the coupler, be disposed between the side of said dielectric layer and having at least one coupling sectionsubstantially one-quarter wave length long at said predetermined frequency, the coupling sections of said first and said second strip conductors being in overlying coupling relationship, a second dielectric layer of predetermined thickness disposed against said first strip conductor, a third dielectric layer of predetermined thickness disposed against said second strip conductor, a first metallic ground plane distric layer of predetermined thickness, a first strip conductor disposed on one side of said first dielectric layer 1 and having acentral and two end coupling sections one end'coupling section of each of saidfirst and said ground planes and be tied, so to speak, to the spacing therebetween. I I

Achieving, the necessary coupling coefficient over the desired band width may involve tightening the cou pling ofonly the'center sections of the two conducting strips, it being highly desirable notto disturb the degree of couplingof theother sections and not todisturb the associated circuitry. The known'prior art is not able to achieve the foregin'g without, "in effect, destroying the coupler. Hence, it is a further object of the invention to provide an improved method or procedure andapparatus which will'overcome the indicated shortcomings of the prior art. v v

It is a further object'of the invention to provide an improved multi section' 'couplerof the nature indicated which will permit the necessary tight coupling of the center section of the coupler while maintaining the ground plane spacing for the rest of the coupler and associated circuitry.

It is a further object of the'invention to provide an improved multi-section-coupler-of the nature indicated which enables a high coupling factor, or coefficient, to be obtained in the center section of the couplerwhile maintaining minimal ground-plane spacing for the rest of the circuitry and a uniform thickness of thecenter dielectric spacer.

SUMMARY OF TI-IE INVENTION t In carrying out the invention in, one form, there is provided .a TEM mode directional couplerv for a predeeach of which is substantially one-quarter wave length long at a predetermiend frequency, a second strip conductor disposed on the other side of said first dielectric layer and having a central and two end coupling sections each of which. is substantially one-quarter wave length long at saidpredetermined frequency, the central coupling sections of said first and said second-strip conductors being in overlying coupling relationship,

second strip conductors being disposed along side each and being in edge'eoupling'relationship, the ether end coupling section of each of said first and said second strip conductors being disposed along side each other and being in edge coupling relationship, a second dielectric layer of predetermined thickness disposed against said first strip conductor, a third dielectric layer of predetermined thickness-disposed against said second strip conductor, a first metallic ground plane disposed against said second dielectric layer, a second metallic ground plane disposed against said third dielectric layer, and means forincreasing the "coupling between said centralcoupling sections of ond strip conductors.

. BRIEF DESCRIPTION OF'THE DRAWING FIG. 1' is a fragmentary, sectional elevational view, taken in the direction of arrows l--1 of FIG. 2,; and on an enlarged scale, of a coupler according to the invention;

FIG. 2-is a sectional view of a different scale taken I substantially in the direction of the arrows 2-2 of FIG.

FIG. 3 is a sectionalview on an enlarged scale taken substantially in the direction of the arrows 3-3 of FIG.

FIG. 4 is a sectional view of an enlarged scale taken substantially in the direction of the arrows 4-4 of FIG. 2;

FIG. 5 is a sectional view of an enlarged scale taken substantially in the direction of the.arrows'5- 5 of FIG.

1 DESCRIPTION OF THEPREFERRED EMBODIMENT I 7 Referring to thedrawings there isshown a TEM dialetric cavity stripling coupler 10 comprising a central said first and said secdielectric layer, or film 1 1, a pair of strip conductors 12 and 13, a pair of dielectric layers 14 and 15, a pair of metallic ground planes, or members, 16 and 17, and a pair of cavities 18 and 19 filled, respectively, with dielectric material 21 and 22.

In one example, the distance between the exterior surfaces of the dielectric layers 14 and 15, dimension b, was of the order of 0.125 inches in a 3dB coupler according to the invention and intended to operate in the frequency range of 2 4 GHz.

Referring to FIG. 2, the central dielectric layer 11 is shown as a sheet, or film, of any suitable dielectric material having a thickness of dimension s, as is well understood in this art.

On the top surface of the dielectric layer 11 there is deposited by any suitable techniques a strip conductor 12, shown shaded, and on the bottom surface of the dielectric layer there is similarly deposited a corresponding strip conductor 13. For example, the conductors l2 and 13 may be deposited on the dielectric layer by well known photo resist and etching techniques. In this manner conductors 12 and 13 may be given any desired width and thickness and may be disposed relatively to each other, as shown, or in any other appropriate position.

The conducting strip 12 includes a terminal section 23, one end section 24, a central section 25, a second end section 26, and a second terminal section 27. The conducting strip 13 includes a terminal section 28, one end section 29, a central section 31, a second end section 32 and a second terminal section 33. The central sections 25 and 31 include a cross-over at 34 necessitated by the fact that the terminal sections 23 and 27 of the conducting strip 12 come out on opposite sides of the coupler and the terminal sections 28 and 33 of conducting strip 13 likewise come out of opposite sides of the coupler. If the terminal sections 23, 27 and 28,

33 were on the same sides, respectively, of the coupler,

the central sections 25 and 31 would not require a crossover, as shown, but would just overlie. The coupler may be madeas desired in this respect depending upon where the terminal sections are to appear.

The dotted line square 35 of FIG. 2 corresponds to the cavity formed by the cavities 18 and 19 of FIG. 1.

The center sections 25 and 31 are parallel to, and overlie, each other as may be seen best in FIG. 3. The coupling between the central sections is by virtue of the overlying arrangement of the conductors. The end sections 24 and 29 are parallel to each other but displaced sidewise as may be seen in FIGS. 2 and 5 to form a gap 36. The coupling between the end sections 24 and.29 is essentially by edge coupling between their adjacent inner edges. The end sections 26 and 32 are parallel to each other but displaced sidewise asmay be seen in FIGS. 2 and 4 to give a gap 37. The coupling between the end sections 26 and 32 occurs by virtue of field coupling between the adjacent inner edges of these coupling sections. Between the various coupling seca couplers of the nature described can, according to and equal to M/4 that is, equal to one-quarter of a wave length at the centerfrequency of the bandwidth for which the coupler is intended to be used.

In an overlay coupler, that is at the center sections 25 and 31 (FIG. 2) the coupling coefficient is proportional to b/s. If in the center setion there were no cavities l8 and 19 (35), the coupling coefficient would then be, of course, proportional to the ratio b/s. The dimensions of either of these quantities can then be changed to alter the coupling coefficient. Changing the dimension 5 would in a completed structure, of course, mean, in effect, destroying it because the strip conductors 12 and 13 are deposited on the dielectric film or layer 11. Changing the dimension b, for example, changing the thickness of the dielectric layers 14 and 15 involves changing the thickness of the whole structure unless the thickness of the ground claim members 16 and 17 is also changed. As may be seen best in FIG. 1 cavities 18 and 19 are fomed, respectively, in the ground planes l6 and 17, the length of the cavity being one-quarter of a wave length of the center frequency )xo/4. The cavities 18 and 19 are filled with dielectric material 21 and 22 which may be of the same material as dielectric layers 14 and 15. Thus under the center section the ratio b/s becomes b'/s and the coupling at the center section is substantially increased. That is the coupling coefficient is increased or the overlay portion of the strip conductors are more tightly coupled. The cavities 18 and 19 may be cut into the inner surface of the ground plane and accordingly do not change the dimensions of the structure nor does this artifice necessitate changing any of the other components of the circuit which, for example, may be disposed on the dielectric film 11. The dielectric pieces 21 and 22 may be small pieces cut to the same shape as the cross-section of the cavities l8 and 19 so as to fit within the openings. The coupling may be adjusted in the manner described after thebasic coupling structure has been made in an effort to achieve flatness of frequency response over the bandwidth desired. The pieces 21 and 22 may be of one piece with the pieces 14 and 15, respectively.

In one stripline coupler constructed according to the invention the maximum unbalance was 0.1 dB, that is v to say 0.05 dB ripple, and the voltage standing wave ratio did not exceed 1.2 to 1. A maximally flat frequency response curve was achieved over an active bandwidth. Thematerial of the dielectriclayer 11 may be of any suitable type, for example, fiber glass impregnated with Teflon and the dielectric layers 14 and 15 may be formed of any suitable dielectric material used in microwave applications. The thickness of the central layer 11 may be of the order-0.008 inches, the conducting strips 12 and 13 are of the order of 0.001 of an inch and the thickness of the dielectric layers 14 and 15 may be of the order of 0.06 inches. The depth of the cavity 18 as well as the depth of the cavity 19 may be of the order of 0.062 inches.

A method, or procedure, for adjusting the coupling the invention, be achieved with a minimum of efiort and a substantially lessening of costs. Greatly reduced throwaway or spoilage is likewise achieved.

We claim:

1. A TEM mode directional coupler for a predetermined frequency range comprising:

a first dielectric layer of predetermined thickness,

a first strip conductor disposed on one side of said firsttdielectric layer and having a central and two end coupling sections each of which is substantially one-quarter wave length long at a predetermined frequency,

a second strip conductor disposed on the other side of said first dielectric layer and having a central and two end coupling sections each of which is substantially one-quarter wave length long at said predetermined frequency,

the central coupling sections of said first and said second strip conductors being in overlying coupling relationship,

one end coupling section of each of said first and said second strip conductors being disposed along side each and being in edge coupling relationship,

the other end coupling section of each of said first and said second strip conductors being disposed along side each other and being in edge coupling relationship,

a second dielectric layer of predetermined thickness disposed against said first strip conductor,

a third dielectric layer of predetermined thickness disposed against said second strip conductor,

a first metallic ground plane disposed against said second dielectric layer, a second metallic ground plane disposed against said third dielectric layer, and

means for increasing the coupling between said central coupling sections of said first and said second strip conductors,

said coupling increasing means comprising cavity means of predetermined depth and length formed in one of said first and said second ground planes adjacent said central coupling sections of said first and said second strip conductors.

2. The directional coupler according to claim 1 wherein said cavity means comprises a first cavity of predetermined depth and length formed in said first ground plane adjacent the central coupling section of said first strip conductor, and a second cavity of predetermined depth and length formed in said second ground plane adjacent the central coupling section of said second strip conductor.

3. The directional coupler according to claim 2 wherein the length of each of said cavities is substantially equal to the length of one of said central coupling sections of said first and second conducting strips.

, 4, The directional coupler according to claim 2 wherein each of said first and second cavities is filled with dielectric material.

5. The directional coupler according to claim 2 wherein the lengths of said first and second cavities is equal to one-quarter of a wave length at the center frequency of the predetermined frequency range. 

1. A TEM mode directional coupler for a predetermined frequency range comprising: a first dielectric layer of predetermined thickness, a first strip conductor disposed on one side of said first dielectric layer and having a central and two end coupling sections each of which is substantially one-quarter wave length long at a predetermined frequency, a second strip conductor disposed on the other side of said first dielectric layer and having a central and two end coupling sections each of which is substantially one-quarter wave length long at said predetermined frequency, the central coupling sections of said first and said second strip conductors being in overlying coupling relationship, one end coupling section of each of said first and said second strip conductors being disposed along side each and being in edge coupling relationship, the other end coupling section of each of said first and said second strip conductors being disposed along side each other and being in edge coupling relationship, a second dielectric layer of predetermined thickness disposed against said first strip conductor, a third dielectric layer of predetermined thickness disposed against said second strip conductor, a first metallic ground plane disposed against said second dielecTric layer, a second metallic ground plane disposed against said third dielectric layer, and means for increasing the coupling between said central coupling sections of said first and said second strip conductors, said coupling increasing means comprising cavity means of predetermined depth and length formed in one of said first and said second ground planes adjacent said central coupling sections of said first and said second strip conductors.
 2. The directional coupler according to claim 1 wherein said cavity means comprises a first cavity of predetermined depth and length formed in said first ground plane adjacent the central coupling section of said first strip conductor, and a second cavity of predetermined depth and length formed in said second ground plane adjacent the central coupling section of said second strip conductor.
 3. The directional coupler according to claim 2 wherein the length of each of said cavities is substantially equal to the length of one of said central coupling sections of said first and second conducting strips.
 4. The directional coupler according to claim 2 wherein each of said first and second cavities is filled with dielectric material.
 5. The directional coupler according to claim 2 wherein the lengths of said first and second cavities is equal to one-quarter of a wave length at the center frequency of the predetermined frequency range. 